HART and FF

Copyright 2004 by ISA – The Instrumentation, Systems and Automation Society. Presented at the ISA 2004, 5-7 October 2004, Reliant Center Houston, Texas, www.isa.org

USING HART® AND FOUNDATIONTM FIELDBUS TOGETHER IN AN INTEGRATED ENVIRONMENT

Alan R. Dewey Principal Product Marketing Manager

Emerson Process Management Eden Prairie, Minnesota 55344

KEYWORDS HART, FOUNDATION fieldbus, Integrated, Architecture, Configuration

ABSTRACT FOUNDATION fieldbus and HART have established themselves as the two major bus technologies used in the process industries. Although HART and FOUNDATION fieldbus enable many of the same benefits such as remote configuration and diagnostics, each technology has some unique characteristics. Because many customers will have a combination of both HART and FOUNDATION fieldbus devices, it is important that the user be able to deal with these devices in as similar a manner as possible. From implementation of control strategies to managing configuration and maintenance data, both HART and FOUNDATION fieldbus devices should present a similar “look and feel” to the customer. This paper will address some of the techniques that can be used to seamlessly integrate both these bus technologies into the same control systems. COMPARING HART and FOUNDATION FIELDBUS Before talking about how HART and FOUNDATION fieldbus devices can be used together in the same system, it is important to understand how these two technologies are similar and how they are different. Devices using HART communication technology were introduced in the early 1980s. The HART Communication Foundation estimates that there are approximately 10 Million HART devices in service throughout the world today. FOUNDATION fieldbus devices started showing up in the mid 1990s. The Fieldbus Foundation estimates that there are approximately 300,000 FOUNDATION fieldbus nodes in service in the world today. From these numbers, it is clear to see that the installed base of HART field devices greatly exceeds those that are using FOUNDATION fieldbus. Fieldbus adoption does continue to

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grow, however. Although some plants are being built today that are virtually 100 % fieldbus, the great majority of FOUNDATION fieldbus devices are being added in plants where HART devices already exist. For this reason, it is important to understand how these two protocols can be used together. For the rest of this paper, it is assumed that fieldbus means FOUNDATION fieldbus. Both HART and fieldbus devices contain a myriad of configuration and diagnostic data. In HART devices, this data is delivered via digital signaling that is “superimposed” on top of the traditional 4-20 ma current loop used to return (or send) the process variable (PV). The communication speed for HART signaling is 1.2 kbps. With HART devices, the PV is almost always derived from the 4-20 ma signal from the device despite the fact that the PV is also available as part of the digital data provided by the device. In fact, many HART devices installed today still use only this PV and ignore the digital data provided by the HART protocol. On the other hand, HART protocol does allow several devices to be connected in series in the same current loop thus providing data digital data from each device. In this case, the current in the loop is kept at a constant value and the PV for each device is obtained from the digital data. Multi-drop HART networks are used in applications where fast update rates are not required. Unlike HART, fieldbus protocol is 100% digital. The communication speed for fieldbus signaling is 31.5 kbps. The concept of a device’s PV being proportional to the current in the loop no longer applies. Fieldbus devices are connected in parallel on a “segment” which must be terminated at both ends. Multi –drop configurations are common with fieldbus. Up to 32 devices can be connected in parallel on one segment (without repeaters). The practical limit is less than this and depends on things like the fieldbus segment’s power supply, intrinsic safety, and the required response time of the devices. These differences are summarized in Figure 1. FIGURE 1 – HART AND FOUNDATION FIELDBUS COMPARISON

Characteristic HARTFOUNDATION

fieldbusUses 4-20 ma for providing PV Yes NoData Provided via Digital Signaling Yes YesType of Communication Master / Slave Token PassingBus Speed 1200 bps 31.25 kbpsRemote Configuration / Calibration Yes YesProvides Device Diagnostics Yes YesMulti-Drop Configurations Yes (limited) YesApproximate Number of Devices in Service 10,000,000 300,000Suitable for Intrinsically Safe Applications Yes YesNon-proprietary / Vendor Neutral Yes YesSupports "Control in the Field" No Yes


ARCHITECTURES USING BOTH HART AND FOUNDATION FIELDBUS Many different architectures are possible for integrating HART and fieldbus. It is beyond the scope of this paper to explore them all. The type of architecture used will depend on many factors such as the capabilities of the control system computer, the ratio of HART to fieldbus devices, and whether the architecture is being “migrated” to support fieldbus (or vice versa). In general, a well integrated on-line architecture would look something like that shown in figure #2. Similar I/O interfaces would connect the HART “loops” and the fieldbus “segments” to the control system computer. In such a configuration, it would be typical for the fieldbus I/O interface to connect to multiple multi-drop fieldbus segments with multiple devices on each segment. The HART I/O, on the other hand, would typically be wired such that each Hart Device connects to a separate channel on the HART I/O interface. In any case , an on-line system, such as that shown in Figure #2, will make available process and diagnostic data from both the HART and fieldbus devices to both the operator and maintenance consoles in a fairly seamless manner. This will be discussed in more detail later. FIGURE 2 – INTEGRATED ARCHITECURE WITH HART AND FIELDBUS DEVICES In some cases, the existing Control System’s I/O Interface may not fully support HART I/O. In this case a 3rd party HART Multiplexer can be used to bring the digital HART messages into the control system as shown in Figure #3. The HART Multiplexer is used to “strip off” the digital HART message and provide it to a software package running in one of the Control System computers.

HART Devices

HART Devices

Fieldbus Devices

Fieldbus Devices

Operator Console Maintenance Console

I/O Interface


FIGURE 3 – PARTIALLY INTEGRATED ARCHITECURE USING HART MULTIPLEXERS In some cases , there may be a desire to integrate HART and FOU NDATION fieldbus devices on the same pair of wires. Although this is not possible directly, it is possible to accomplish something like this by using “gateway” type devices. This may be the case if you have a fieldbus segment in a particular area of the plant where you want to re-use an existing HART device without having to run separate wiring. Another example might be where you simply cannot find a fieldbus device to make the measurement you want so you are forced to use a HART device for a particular measurement. In cases like this, you would most typically use a current to fieldbus input converter. In a similar way, you might want to re-use a HART valve on a fieldbus segment. Here you would use a fieldbus to current output converter. Although the use of these converters can be a good temporary solution, the disadvantage is that you are no longer bringing the HART device’s configuration and diagnostic data into the system. Only the PV of the device is read or written. Some gateway converters on the market do attempt to map some of the HART configuration data into a pseudo fieldbus Transducer Block but these gateways tend to be very device specific.

HART Devices

HART Devices

Fieldbus Devices

Fieldbus Devices

Operator Console Maintenance Console

I/O Interface

HARTMultiplexer

HARTMultiplexer


MANAGING CONFIGURATION DATA FROM HART AND FIELDUS DEVICES HART and fieldbus devices both have similar configuration and diagnostic data contained in them. Both HART and FOUNDATION fieldbus use the same Device Description Language to define their contents. Unfortunately, this is where the similarity ends. When HART was first introduced, the primary means of configuring these devices were handheld computers and simple PC tools. This drove the HART device manufacturers to develop Menus for their devices using the DDL language. These Menus were used to organized configuration and diagnostic data into logical groupings such as Basic Setup, Detailed Setup, Range, etc. These text based menus worked well with handheld communicators. When more sophisticated PC configuration and Asset Management packages were introduced, suppliers of these programs usually required that a different file be developed for each device to better make use of the color graphic capabilities of a CRT screen. Although roughly based on the menus that were developed for handhelds, the configuration files used for the PC programs provided the device’s configuration and diagnostic data in a much more user friendly manner. When the FOUNDATION fieldbus protocol was developed, device parameters were organized into “blocks”. Parameters in a fieldbus device are organized as follows:

• One Resource Block containing general identity information about the device • One or more Transducer Blocks containing configuration and diagnostic information specific to

that device • One of more Function Blocks for implementing “control in the field” on the fieldbus • Other misc. device parameters such as tag, node address, etc

Suppliers of fieldbus Control Systems and Configuration Tools usually display a fieldbus device’s parameters organized by the above block types. Within each block type, the parameters are either organized alphabetically or in accordance with a special vendor specific file provided for that device. Again, this special file is intended to organized the parameters in a logical manner and make use of the color graphics capabilities on the Control System’s console. Because no handheld configurators for fieldbus were available when it was introduced, there was less incentive for device suppliers to develop DD Menus as was described for HART above. Therefore, the device’s registered Device Description is less likely to provide a logical organization of a device configuration and diagnostic data in a FOUNDATION fieldbus device than in a HART device. This makes the vendor specific file for formatting this information even more important for fieldbus devices. This need to provide a standard way to organize parameters for both HART and fieldbus devices is recognized by the industry. A joint committee consisting of representatives from the Fieldbus Foundation, the HART Communication Foundation and the Profibus Nutzer Organization have recently released a specification for extensions to the Electronic Device Description Language (EDDL). The EDDL extensions provides a way of grouping parameters in the device’s Device Description without the necessity of special formatting files for the control system. Figures 4 and 5 give an example of the similarities and differences in displaying FOUNDATION fieldbus devices and HART devices on the same User Interface on an integrated system.


FIGURE 4 – DISPLAY OF A FIELDBUS DEVICE IN AN INTEGRATED SYSTEM Figure 4 shows a device “tree” containing both HART and fieldbus devices. This explorer type view is typical of many control systems. In this example, the configuration properties of a particular fieldbus device have been selected. The left hand side of the window shows the Resource and Transducer blocks that are contained in that device. The first Transducer Block has been selected. Within that Transducer Block , the parameters are further organized by Process, Sensor, Primary PV, and Secondary PV. The configuration parameters related to the device’s sensor are shown. Figure 5 shows a similar display for a HART device. Again, the HART device is selected from the explorer “tree” just like the fieldbus device. Because there are no Resource Blocks and Transducer Blocks in HART device, the parameters are organized slightly differently. Still, many of the parameters associated with the device’s sensor are similar. Ideally, HART and fieldbus device suppliers will make the organization of these parameters as similar as possible so that finding the right device parameter is intuitive for the user.


FIGURE 5 – DISPLAY OF A HART DEVICE ON AN INTEGRATED SYSTEM INTERGRATED CONTROL STRATAGIES USING HART AND FIELDBUS DEVICES One fundamental difference between HART and FOUNDATION fieldbus devices is that control strategies can be implemented in the field devices themselves with FOUNDATION fieldbus devices. The same control strategies can be implemented with HART devices , of course, but the actual control algorithms themselves would be executed in the control system computer or PLC. A well designed control system will allow an integrated control strategy to be use employing devices independent of their communication protocol. The following example illustrates this concept. Figure 6 shows a simple cascade control loop.


FIGURE 6 – CASCADE CONTROL LOOP USING BOTH HART AND FIELDBUS DEVICES In this example, FT101 and FCV101 are FOUNDATION fieldbus Function Blocks located in fieldbus devices FT-101 and FCV-101, respectively. TT100 and TIC100 are non-fieldbus Function Blocks located in the Control System PC. TT-101 is a HART device providing the input to the TT100 AI Function Block. The trick is for the Control System to convert the output of the conventional TIC100 Function Block into a fieldbus output so that it can provide a cascade input to the FIC101 fieldbus function block contained in fieldbus device FT101. A well designed control system will provide tools that make the above example relatively seamless to the user. The diagram shown in Figure 7 shows how a user might setup the above control strategy using both FOUNDATION fieldbus and HART devices. Notice that the function blocks in the “HART” loop (i.e.AI1 and PID1) loop look exactly the same as those in the “fieldbus” loop (i.e. AI2, PID2, and AO1). The only difference is that the input block in the HART loop (i.e. AI1) is identified with the Host Control System’s I/O card and channel number while the input and PID blocks for the fieldbus blocks are identified by the Device Tag of the fieldbus devices in which these blocks are located.

TT100

TIC100

FT101

FIC101 FCV101CAS

IN

IN

SP

Product

SteamHart

I / O

FFI / O

FT-101

FCV-101

TT-101

Control System


FIGURE 7 – CONTROL STRATEGY CONFIGURATION USING BOTH HART AND FOUNDATION FIELDBUS DEVICES DIAGNOSTIC ALERTS IN AN INTEGRATED ENVIRONMENT Both HART and FOUNDATION fieldbus devices have the capability of providing a wide range of diagnostics data about the device’s health. In general, the diagnostic capabilities of HART and FOUNDATION fieldbus devices are about the same. The types of device diagnostics vary widely depending on the type of device. Measurement transmitters will have a lot of diagnostics related to the status of the transducer and measurement logic in the device. Control devices such as valves will provide a lot of information about the mechanical condition of the device. Both transmitters and valves will provide diagnostics information about the communication electronics in their respective devices. Although the diagnostic data provided by HART and FOUNDATION fieldbus devices is very similar, the way in which they are delivered to the control system and presented to the operator or technician can be quite different. The reason for this has to do with the speed and characteristics of the communication


technology used by these two protocols. FOUNDATION fieldbus uses point to point communication technology. This means that , when a fieldbus device detects a diagnostic condition that it wants to report, it can send an event out on the bus with the relevant information. This event is picked up by the control system and can be immediately displayed or annunciated on the console. HART devices, on the other hand, have to be continually “polled” to see if there is anything to report. Because the polling is done at 1200 BPS with HART, there are limitations on how many devices can be “polled” for alerts in a specific timeframe. A small number of critical devices can be polled for alerts very quickly (i.e within seconds) or a large number devices can be polled within minutes. Nevertheless, it is possible to implement an effective diagnostic alert system with HART as long as the restrictions on response and device count are understood. Once the operator or maintenance person becomes aware of a problem in a field device either via an alert or some other means, the actual display of the status information from both HART and fieldbus devices is very similar. Often times, a record of this status event will automatically be logged in the control system. Again, the logging of HART and FOUNDATION fieldbus device problems should normally look the same on a well integrated system. INTEGRATED MAINTENANCE TOOLS So far, we have discussed primarily how HART and FOUNDATION fieldbus devices can be configured and diagnosed on the same control system. It is also important to consider what type of portable maintenance tools one needs in systems that contain both HART and FOUNDATION fieldbus devices. Portable tools currently fall into two general categories. The first are intrinsically safe handheld devices. Several of these exist for HART only devices. Recently, a combined HART and FOUNDATION fieldbus intrinsically safe handheld device has become available. This integrated tool allows the user to configure and diagnose both HART and fieldbus devices while in the field. The second type of portable tools are designed around laptop or notebook computers. Ruggedized versions of these portable computers can make an effective tool. The only disadvantage is that a separate interface is needed for HART and FOUNDATION fieldbus. And, of course, these types of computers cannot be used in hazardous (i.e Class 1 Div 1) areas of a plant. Advances are being made in small handheld computers. Some of these are now even classified for Class 1 Div 1 areas. How practical these will be in a plant environment remains to be seen.

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HART and FF